A known caustic soda (NaOH) manufacturing process is carried out by reacting dissolved soda ash (Na.sub.2 CO.sub.3) with hydrated lime (Ca(OH).sub.2) to produce caustic soda and a precipitated calcium carbonate byproduct. The reaction occurs in an aqueous slurry at a dilute concentration, generally 10%. The reaction mechanism for the lime-soda process is: ##STR1## See Hultman et al. U.S. Pat. No. 4,311,666, issued Jan. 19, 1982. The reaction temperature and residence time are adjusted to maximize the NaOH yield. Soda ash is available from a variety of sources, and may be recovered from aqueous sodium carbonate solutions by a method involving evaporation and centrifugation as described in Copenhafer et al. U.S. Pat. No. 4,519,806, issued May 28, 1985.
Calcium carbonate is used in the paper industry for paper filling and coating applications in the form of a slurry of fine calcium carbonate particles in water, with other known additives. See, for example, the calcium carbonate pigment described in Japanese Pat. No. 82-30814. It is known to use a neutralized copolymer of (meth)acrylic acid and maleic or fumaric acid as a stabilizer in such a composition; see Japanese patent publication 53144499 (1978).
Calcium carbonate is both mined and made synthetically by a variety of well known processes. See, for example, Heytmeijer et al. U.S. Pat. No. 4,100,264, issued Jul. 11, 1978, wherein calcium carbonate is produced from calcium chloride, and Faatz U.S. Pat. No. 4,272,498, issued Jun. 9, 1981, which discloses a method of making a fine calcium carbonate slurry by reaction with carbon dioxide. Pettersson U.S. Pat. No. 4,941,945, issued Jul. 17, 1990, concerns a method of precipitating calcium carbonate by treatment of green liquor with caustic lime. Japanese patent publication 62-171921 produces a fine calcium carbonate suitable for use a paper filler by use of a gas-liquid plane contact reactor.
Other processes have been proposed for recovering calcium carbonate produced by the lime-soda reaction and other reactions in cake form. See Japanese patent publication 73-9720 (also recovering NaOH), and Soviet Union patent publication Nos. 490,755, 522,136, 560,829, and 691,401. Use of lime muds obtained during causticisation of Kraft paper pastes to recover CaCO.sub.3 for use as a paper pigment has been proposed in French Pat. No. 2,570,721 (1986). French Pat. No. 2,544,353 (1984) recovers CaCO.sub.3 by treatment of liquor for causticisation.
Despite the many methods available for the production of calcium carbonate and sodium hydroxide, a need remains for a process which can efficiently produce calcium carbonate of a grade suitable for paper industry applications, particularly in combination with sodium hydroxide also useful in paper making processes. The present invention addresses this need.